In recent years, a compact portable electronic device such as a digital camera or a mobile phone has been widely used. In these electronic devices, it has been demanded to minimize the volume and to reduce the weight all the time. In a battery mounted thereon, it has been also demanded to realize a small, lightweight, and high capacity battery. Particularly, a lithium ion secondary battery obtains a higher energy density than a water-soluble secondary battery such as a lead storage battery, a nickel cadmium battery, or a nickel hydrogen battery, and therefore importance thereof as a power source for a personal computer, a portable terminal, or the like has increased. Furthermore, the lithium ion secondary battery is expected to be preferably used as a high output power source mounted on a vehicle.
The lithium ion secondary battery has an advantage of a high energy density, but uses a nonaqueous electrolyte, and therefore sufficient countermeasures against safety are required. In recent years, securing safety has become a big problem in accordance with a larger size of a battery and a higher capacity thereof. For example, when the temperature of a battery rises abnormally and rapidly due to overcharge, short circuit inside the battery, or the like, it is difficult to regulate heat generation only by a safety mechanism provided outside the battery, and there is a risk of ignition.
Patent Literature 1 describes a method for bonding an electron conductive material having a function of a positive temperature coefficient resistor (hereinafter, referred to as PTC) to a current collector. However, the thickness of the electron conductive material is as thick as 50 μm, and therefore an energy density as an entire battery is lowered. Therefore, this method is not preferable.
Patent Literature 2 discloses that PTC characteristics are imparted to any one of a positive electrode, a negative electrode, and a nonaqueous electrolytic solution. However, in order to impart PTC characteristics to these, it is necessary to add a large amount of additives not contributing to a battery capacity, resulting in a decrease in energy density.
Patent Literature 3 describes a method for providing a conductive layer formed of a crystalline thermoplastic resin, a conductive agent, and a binder on a surface of a current collector. When the temperature inside a battery exceeds a melting point of the crystalline thermoplastic resin, resistance of this conductive layer rises, and a current between the current collector and an active material is cut off. However, internal resistance during normal operation of the battery increases, and output characteristics of the battery are lowered. Therefore, this method is not preferable.
Patent Literature 4 describes a method for providing a conductive layer formed of polyvinylidene fluoride and a conductive agent on a surface of a current collector and heating the current collector provided with this conductive layer at a temperature exceeding 120° C. However, a step of a heat treatment is added, and an increase in resistance when the temperature inside a battery rises is not sufficient. Therefore, this method is not preferable.
Patent Literature 5 describes a method for providing a current collector provided with a conductive layer formed of conductive particles, carboxymethyl cellulose, a water-dispersible olefin resin, and a dispersant. However, carboxymethyl cellulose is used as a thickener for a dispersion, and the addition amount thereof is as small as 5% by mass or less in the total solid content of 100% by mass of the dispersion.
Patent Literature 6 describes a method for providing a conductive layer formed of a composition in which a volume average particle diameter of heat fusible particles is larger than a volume average particle diameter of conductive inorganic particles. However, an increase in resistance when the temperature in a battery rises is not sufficient. Therefore, this method is not preferable.
Patent Literature 7 describes a method for providing a current collector provided with a conductive layer formed of an aggregate of polyolefin emulsion particles using a polymer flocculant and/or a low molecular flocculant and a conductive material. However, it is difficult to control an aggregation diameter of an emulsion particle using a polymer or low molecular flocculant, and an effect thereof is not sufficient. Furthermore, a surface of a conductive material is covered with a nonconductive polyolefin resin. Therefore, internal resistance during normal operation of a battery increases, and output characteristics of the battery are lowered. Therefore, this method is not preferable.